Electrical motors are widely used for many different applications and are commonly used in domestic appliances. For example, in a vacuum cleaner a motor is used to drive a fan that causes dirty air to be sucked through a dirty air inlet. The dirty air passes through some form of separation device such as a cyclonic or bag separator that separates dirt and dust from the airflow, and finally the air is exhausted from an air outlet.
Switched reluctance machines have become increasingly popular in recent years. In a switched reluctance motor, a stator has sets of poles that are sequentially energised to rotate a rotor into line with the energised pair of poles, under the influence of the magnetic fields associated with each set of poles. By rapidly switching between different pairs of poles, it is possible to cause the rotor to rotate at a very high speed.
Switched reluctance machines have an advantage in that they do not use carbon brushes, which need to be replaced periodically and which emit particles of carbon into the atmosphere as they wear down. Furthermore, the motor has a relatively long life and its speed is not limited by the need to maintain a reasonable brush life.
In order to energise sequentially the stator poles, the position of the rotor with respect to the stator poles has to be determined with great accuracy. It has been proposed to use an encoder disk mounted on the shaft. The encoder is arranged, in use, to rotate with the shaft and interrupt intermittently light travelling between a transmitter and a detector. Thus, during manufacture, the encoder has to be mounted carefully on the shaft in a predetermined position and orientation with respect to the rotor so that the rotational position of the rotor can be determined. Alternatively, the encoder can be mounted on the shaft in an arbitrary orientation, which then must be determined with respect to the rotor before use.